Alternating current (AC) induction motors use a large proportion of a given industrialized country's generated electrical power. It is estimated that roughly 70% of the United States' total generator power is used to power motors. To optimize power consumption, it is often desirable for a motor selected for a given application to be able to drive the largest possible load at the lowest possible line voltage. The relative efficiency of an AC motor may be expressed in terms of a power factor that is related a difference in phase between an AC voltage applied to the motor and an AC current utilized by the motor. The power factor is sometimes expressed as a cosine of the relative phase angle between the AC source voltage and the AC motor current. When the source voltage and motor current are in phase, the phase angle difference is equal to zero and the cosine of the phase angle is equal to 1.
In an AC induction motor, fed power directly from the source, will run optimally (e.g., power factor close to 1) only in the situation where the AC motor has the largest possible load and the source powering the motor is operating at the lowest possible line voltage. As soon as the line voltage begins operating higher than the minimum possible voltage, or the mechanical load is lower than maximum possible load, the motor's power factor will be less than optimal. The average power factor of AC motors used in the United States is about 0.7. This means that as much as 30% of a source's generated power could be lost due to the less than optimal power factor of the AC induction motor. If one considers that approximately 70% of the electric power generated in the United States is derived from fossil fuels, such as coal, natural gas and oil, there is a great need to optimize the operation of all AC induction motors given the current limited nature of the supply of such fuels.
The best way to optimize the power factor is to reduce the motor's supply voltage such that it is proportional to the instantaneous mechanical load of the AC induction motor. The earliest solution to this problem involved compensating a voltage lagging phase-dislocated current driving an AC induction motor with a leading current. This method could improve the power factor from the power supply's point of view, but at the same time this method caused a large current to exist between the motor and the circuit used for compensation. Because of the high costs, low efficiency, and high maintenance of this procedure, this approach was never widely adopted.
Another solution to optimizing the power factor in AC induction motors was developed by Frank Nola, who developed a power factor control system for use with AC induction motors. Nola's power factor control system samples line voltage and current through the motor and decreases the power input to the motor in proportion to the detected phase displacement between the current and voltage (see for example U.S. Pat. Nos. 4,052,648, 4,433,276, and 4,459,528, which are sometimes referred to herein as the Nola patents). This method reduces the power to the motor, as it becomes less loaded. Although Nola's power factor correction method was a big step forward, it had its basic problems. According to Nola's patent, the power of the motor is controlled by silicon-controlled rectifiers, which will turn on following a delay after the zero crossing of the input voltage. The motor's power reduction is proportional to the phase difference between the last zero crossing of the input voltage and the moment of turn on of the silicon controlled switch. For low power factor errors, the system works reasonably well, but as soon as the power factor errors become large, the waveform of the motor current becomes severely distorted. The result is the emergence of harmonics on the input line frequency. Harmonics of a third order will cause the “lifting” of a neutral line, which is unacceptable due to the dangers that it poses.
U.S. Pat. No. 6,194,881 to Parker et al. discloses a switching power supply system that includes first and second AC switches which are operated at alternate intervals with respect to each other to permit current to flow between the AC power line source and the load over intervals of the AC voltage cycle. The system includes an energy storage element (e.g., an inductor) in an output filter that stores energy during intervals of the AC voltage cycle and releases the stored energy during the alternate intervals of the AC voltage cycle. Since the switches are turned on alternatively the timing of the switches is critical to avoid current overlapping or open circuit. For example, if both switches are closed (i.e., “on”) there will be a short between the live and neutral lines. If the second switch opens before the first one closes an inductive ‘kick’ back voltage from the inductor could destroy both switches. Furthermore, the circuit loss is relative high since at any time there are four diodes and two switching elements in the current path.
U.S. Pat. No. 5,635,826 to Sugawara describes an AC power source system that is similar to the one disclosed by Parker in U.S. Pat. No. 6,194,881. In the Suguwara system, a first AC switch provided between input and output sides is on-off operated in a predetermined cycle. A second AC switch is provided on the output side of the first AC switch at a position to short-circuit the output side and on-off operated conversely to the first AC switch. A predetermined pause time is provided between the operations of the first and second AC switches. Each AC switch has two semiconductor elements, and diodes each connected between controlled terminals of and in opposite conduction polarity to each semiconductor element. Like polarity controlled terminals of the two semiconductor elements are connected to each other. The same control signal is supplied to control input terminals of each semiconductor element for on-off switching AC between the other controlled terminals of the semiconductor elements. Like the system described in U.S. Pat. No. 6,194,881, the Sugawara circuit uses alternating switching. Therefore timing of the switching is critical to avoid current overlapping or open circuit. The Suguwara device uses passive dissipative components in a snubber circuit to limit surge currents. However, the passive dissipative components in the snubber circuit tend to limit the efficiency of the circuit.
It is within this context that embodiments of the present invention arise.